You can't have different outputs at the same time, even with multiple video cards. You can, however, set up multiple media extenders and run them off the HTPC via a network connection. Check out this site for the ultimate Media Center Home Sports Bar setup:

There's difficulty, but yes you can. I have had TMT on 1, PDVD on 2 and Autumn Wave GT on a third.
Also audio it seems is not that difficult as long as you have displays enabled.http://forums.guru3d.com/showthread.php?t=337698
As long as the tuner is capable of dual output. It is a PAIN as I have found no way of easy restart. Each time you have setup each display to output manually.

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS

not different outputs, but I'm looking more for having, for instance, 2 windows open in windows running 2 different channels off of 2 different tuners, on 1 OS. Also, I'm looking for ways to use my current setup without having to purchase extenders

What do you mean I can't have different outputs running at the same time? Multi-monitor setups have been around for ages, no?